Diplex filters are well known in the prior art and are used to allow simultaneous passage of two electrical signals through the same transmission medium. U.S. Pat. No. 5,390,337 describes one example of a diplex filter and is incorporated herein by reference.
Diplex filters are typically used, for example, in radio frequency communications systems where the same antenna is used for both receiving and transmission. Diplex filters also have application in bi-directional repeater and land line systems such as two-way broadband communications systems and the like.
A diplex filter is usually a three-port passive device, as illustrated in FIG. 1. The diplex filter has an input terminal 1 for receiving an input signal in the "A" direction, an output terminal 2 for providing an output signal in the "B" direction and an input/output terminal 3 for bi-directional signals in the "A" and "B" directions. The diplexer includes a high-pass filter 4 and a low pass filter 5 which provide isolation between the bi-directional signals. High-pass filter 4 is connected between input terminal 1 and input/output terminal 3. Low-pass filter 5 is connected between output terminal 2 and input/output terminal 3.
When properly constructed, high-pass filter 4 passes energy within a first designed band pass between input terminal 1 and input/output terminal 3 with minimum attenuation while providing a high degree of attenuation for energy outside of the band pass. Low-pass filter 5, on the other hand, passes energy within a second designed band pass with minimum attenuation while providing a high degree of attenuation for energy outside of the second band pass. In summary, each filter circuit within the diplexer provides a high Q passband for one band of frequencies while rejecting energy at frequencies outside the band.
While diplex filters play an important role in providing efficient and cost effective communications, they also cause some attenuation of the signals within the pass band due to insertion loss. Thus, the design and use of a diplex filter often requires tradeoff between adequate isolation between the two bands of frequencies of interest and acceptable insertion loss.
A diplex filter has more insertion loss at the band edges than in the middle of the passband. This characteristic insertion loss causes a frequency response variation, or roll-off, that becomes significant when the filters are cascaded, e.g., in two-way broadband communications system. Roll-off is generally defined as a gradual increase in signal loss or attenuation with an increase or decrease in frequency of the signal beyond the substantially flat portion of the amplitude-frequency response characteristic of the system.
Such roll-off at the band edges is illustrated in FIG. 2. FIG. 2 is an amplitude-frequency response graph which compares the amplitude of the pass signal 5 through the filter with its frequency. Ideally, all signals within the pass band will pass through the filter with minimum attenuation and without amplitude variation due to frequency. As pointed out above, however, signal frequencies near the edges of the pass band will suffer greater attenuation as illustrated by the roll-off area shown in FIG. 2.
In the prior art, the effects of insertion loss are addressed with a trim or frequency compensation circuit. The compensation circuit compensates for the roll-off with the goal being a flat frequency response for the system. While the use of trim circuitry decreases the amount of roll-off, the trim circuit itself adds insertion loss to the system. Moreover, each pass band of frequencies, i.e., frequency split, requires a different trim circuit. The need for different trim circuits for each frequency split, usually results in a compromise to the different roll-off characteristics.
The general problem of signal attenuation in a cable distribution system, for example, is addressed in U.S. Pat. No. 4,087,762 which issued in the name of Albert H. Ashley.
The Ashley patent is specifically directed to a cable equalization resonant amplifier circuit which includes circuitry for compensating for the effects of signal attenuation in long coaxial cable runs. The patent teaches locating the amplifier circuit at the receiver end of the cable run and matching the gain of the amplifier with the attenuation characteristics of the cable over the bandwidth of signals being amplified by the amplifier. The Ashley patent does not, however, address the above described problem associated with the use of diplex filters, especially in a two-way broadcast distribution system.
Thus, there is a need in the art for an improved method and apparatus of reducing and/or eliminating the effects of insertion loss attributed to diplex filters.